Except it obviously IS new hardware, hence he calls them "the new torpedoes."

That would be like saying "the new torpedoes for our submarine have arrived. They have 11% more Torpex, and I have personally tuned the passive sonar. Oh, and they now work by hanging from hot air balloons pushed by wind, but I will not mention that at all because it is not particularly relevant."

Actually it would be a bit like the first live-fire tests of an ERGM shell. New guidance system, booster stage in the shell. Neither of which really rates mention since the entire crew would have been briefed on them hours before the test.

If the "Genesis" torps indeed are rocket-propelled guided missiles

Doubtful they're even that. Probably just a standard photon torpedo with a more complicated payload wrapped up in the bolt to increase accuracy and yield. Didn't seem to work all that well.

So you can come up with no explanation whatsoever as to why Spock and McCoy would install the sensor in the torpedo, as opposed to leaving it where it originally was?

Already did. The guidance system for the torpedo is in the casing, which has the physical connection to the energy bolt.

As for "leaving it where it was," I doubt the sensor would have done much good if they left it in a storage locker somewhere.

Epic fail. ST6:TUC is sufficient proof for the missile nature of the weapon

Except... where's the torpedo, then? Invisible?

Nog works on a torpedo of unknown type

Which fire with the distinct and non-twinkly blue-white glow associated with quantum torpedoes.

Why should this matter?

Because physical projectile weapons -- rockets, missiles, spatial torpedoes, etc -- appear to have much longer flight times up to and including long-range interplanetary cruise modes. Photon torpedoes are a distinct type of weapon used at considerably shorter range, typically in either ship to ship or ship to surface modes; they do not cruise any distance and move at much higher speeds, despite the fact that they do not accelerate noticeably after leaving the tube. They are, IOW, more similar to bullets than missiles.

After the 22nd century, photon torpedoes continue to evolve, becoming (arguably) more accurate, more sophisticated, more powerful. Spatial torpedoes, however, fall out of use, despite the apparent portability of photon warheads, despite the improved propulsion technologies (indeed, a 24th century spatial torpedo would be essentially a Class-8 probe with a tricobalt device in the warhead). So for whatever reason, Starfleet does not appear to carry straight projectile weapons anymore.

When you consider the famous Voyager Torpedo problem, it's easy to understand why. Photon torpedo casings may be expendable with the intention of being recycled and properly disposed of after firing (like shell casings on a modern naval gun), but if you really need to and had a lot of time on your hands you could probably regenerate those casings and use them again later.

if a Class 8 Probe can fly at warp without glow, what excuse does a photon torpedo have for not doing the same?

The fact that the glow IS the torpedo; it's the thing that makes it a torpedo in the first place. If it wasn't glowing, it wouldn't be a photon torpedo (at least, we've never SEEN one that didn't).

I'm not even questioning the viability of missile weapons in Star Trek. I'm drawing both on backstage evidence that photon torpedoes were not originally meant to BE missile-type weapons, but a totally distinct technology similar but with slightly different properties as phasers. Physical casings made their debut in TWOK and appeared again in TUC; both times can be made consistent with the original TOS conception, and that would explain a lot of OTHER things that don't make sense about photons.

Because then you won't have to use any energy from the bolt to heat up the payload.

What does that mean? If "the bolt" is your supposed abstract ball of destructive light, what is "the payload"?

The payload can be whatever you want it to be, including nothing (in which case it would be just as deadly, depending on the yield). Like a naval gun round that can be loaded with high explosives, submunitions, smoke canisters, napalm, or nothing at all, and still fired to hit a target.

And yet, the casing hardly needs to be IN SPACE to do that.

Obviously it does. The casing is the only way to send the sensor to space.

The WHOLE SHIP is in space, as is the launch tube where the torpedo comes from. It makes no difference.

As for the visuals, they are always large enough to hide a runabout, especially in the picture you linked to.

Incorrect, demonstrably so from "Maquis ptII" where photon torpedoes flying past a runabout have a glowing core visibly smaller than the runabout's bussard collector.

The bright red core of the torpedo isn't much larger than a torpedo deck window -- one of these, in other words -- and is therefore about the size of a beachball. Even the torpedo from TMP has a bright central core and an outer plume that would barely conceal a travel pod. The torpedo casing should be plainly visible in front of it from at least the forward angle, if it's intended to be there at all. Even the VFX artists don't seem to believe there's any PHYSICALITY to the weapon in flight; it's effectively a phaser blast rolled into a shiruken and accelerated towards the target by a sophisticated (and forward-fixed) cannon and some rudimentary physical guidance mechanism.